The present invention relates to a vehicle air conditioner.
A typical refrigerant circuit in a vehicle air-conditioner includes a condenser, an expansion valve, an evaporator and a compressor. The compressor is driven by a vehicle engine. The compressor draws refrigerant gas from the evaporator, then, compresses the gas and discharges the compressed gas to the condenser. The evaporator performs heat exchange between the refrigerant in the refrigerant circuit and the air in the passenger compartment. The heat of air at the evaporator is transmitted to the refrigerant flowing through the evaporator in accordance with the thermal load or the cooling load. Therefore, the pressure of refrigerant gas at the outlet of or the downstream portion of the evaporator represents the cooling load.
Variable displacement compressors are widely used in vehicles. Such compressors include a displacement control valve that operates to maintain the pressure at the outlet of the evaporator, or the suction pressure, at a predetermined target level (target suction pressure). The control valve feedback controls the displacement of the compressor by referring to the suction pressure such that the flow rate of refrigerant in the refrigerant circuit corresponds to the cooling load.
However, when the engine speed is changed and the flow rate of refrigerant is changed, accordingly, the displacement of the compressor may not be immediately changed. For example, if the engine speed and the flow rate of the refrigerant are increased when the thermal load of the evaporator is high, the compressor displacement will not be decreased until the actual suction pressure falls below the target suction pressure. Therefore, as the engine speed increases, the mechanical work for driving the compressor increases, accordingly, which lowers the fuel economy.
Accordingly, it is an objective of the present invention to provide an air conditioner that permits a variable displacement compressor to maintain a target control value for controlling the compressor displacement without being affected by thermal load of an evaporator.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an air conditioner including a refrigerant circuit is provided. The refrigerant circuit has a variable displacement compressor. A first pressure monitoring point and a second pressure monitoring point are located in the refrigerant circuit. The second pressure monitoring point is downstream of the first pressure monitoring point. The air conditioner includes a displacement control mechanism, which controls the displacement of the compressor based on the pressure difference between the first and second pressure monitoring points such that the pressure difference seeks a predetermined target value. The first pressure monitoring point or the second pressure monitoring point is arranged in the refrigerant circuit such that a pressure that includes the dynamic pressure of refrigerant in the refrigerant circuit is introduced into the displacement control mechanism through the first pressure monitoring point or the second pressure monitoring point.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.